Radio base stations used in mobile telephone communications systems emit large quantities of heat, which must be removed. Typical cabinet heat emissions may be as large as 1600-2500 W. Up to 80% of this heat emission takes place in the radio transmitter equipment. The cooling function is instrumental in controlling the mechanical design of the cabinet more than the radio or electronic equipment. Contaminated and wet outdoor air cannot be used directly to cool the electronics. The outdoor air, or ambient air, however, is the ultimate receiver of the heat emitted from the cabinet.
Hitherto, radio base stations have mostly been installed indoors. The mechanics and the electronics and radio equipment cooling system have been designed to fulfil the requirements occasioned by indoor climates, both with regard to temperature and the concentration of air-carried contaminants. However, there is a trend in the overall manufacture of mobile telephone systems to construct small outdoor installations which are mounted on posts or on the outer walls of building structures and used in the cellular network. In principle, these outdoor installations have hitherto been indoor systems mounted outdoors in containers that include large cooling machines. It is probable that outdoor installations will become more and more usual in the future, particularly in the case of smaller installation types referred to as compact, micro or pico radio base stations. This will place new requirements on the cooling system and on the mechanics.
Essentially three different types of outdoor installations are used at present, viz direct cooling with outdoor air, hermetically sealed spaces equipped with heat-exchangers, and hermetically sealed cabinets equipped with air-conditioning, cooling machines.
The first type of installation includes an outdoor cabinet which houses electronic equipment constructed in the indoor mode, and is cooled directly with outdoor air that has been filtered very effectively. Although this filtering process removes solid contaminants, it does not remove wet and acid gases, which have a damaging effect on the electronics.
In the second type of installation, the heat emitted is released to the ambient outdoor air via heat-exchangers. In this case, the heat emitted, which can reach several kW, must pass through several stages before being dispersed to the ambient air: From component to heat sink, to air within the housing or cabinet, through heat-exchangers, and finally out to the ambient air. Each stage or step results in a certain increase in component temperature. The heat-exchanger can result in a 15.degree. C. increase of indoor temperature contra outdoor temperature. In climates of 45-50.degree. C., this design will be unable to fulfil the requirement of maintaining electronic and radio components at a temperature of 70-80.degree. C. The heat-exchanger must also be designed for all heat that passes therethrough. In the case of capacities of one to two kW (1-2 kW), this means that the heat-exchanger must have a size comparable with or almost equal to the size of the electronic and radio equipment.
The cooling equipment which includes cooling machines, and possibly also heat-exchangers, used in the third type of installation, may have a size which is as much as half the size of the radio and electronic equipment. Redundancy cannot be easily achieved. The system is highly complex and requires an advanced control system. The present cost of a system that lacks redundancy may reach to one-third to one-half of the cost of the entire radio base station. Operating costs are just as high. More than one-third of the heat emission power, in addition to that generated in the electronic equipment, is needed for the cooling machine to remove heat from the housing. This additional heat is liable to influence the surroundings in some sites or scenario. However, this type of cooling system is the only available type that will fulfil the requirements that are placed on the temperature of the electronics with regard to both indoor and outdoor climates.
The aforesaid systems have the drawback of being space-consuming and complicated, of requiring additional fans to circulate air through the system, and of incurring high operating costs.